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29
Brazed Heat Exchangers for CO 2 systems Design of Heat Exchangers for Heat Recovery in Transcritical CO 2 systems Rolf Christensen
Transcript of Brazed Heat Exchangers for CO2 systems - … · Agenda • Alfa Laval shortly.. • BHE Products...
Brazed Heat Exchangers for CO2 systems
Design of Heat Exchangers for Heat Recovery in Transcritical CO2 systems
Rolf Christensen
www.alfalaval.com
Agenda
• Alfa Laval shortly..
• BHE Products for CO2 Cascade • The transcritical process in PH and TH diagrams
• LMTD and Heat Exchanger Design models
• The consequences of physical properties in transcritical state
• Internal pinch point • Minimum operating pressure and outlet temperature • Impact on heat exchanger design
• Some Do’s…
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– a global company
Focus in refrigeration • Energy efficiency • Heat recovery • Heat pumps • Natural refrigerants
– CO2, NH3 and HC • Complete product range
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www.alfalaval.com © Alfa Laval Slide 5
CO2 as low temp refrigerant
R744 (CO2)
R134a
2,2 bar (-8 Deg.C)
8,9 bar (35 Deg.C)
30,5 bar (-5 Deg.C)
19,7 bar (-20 Deg.C)
CO2 Cascade
Cold room
R744 (CO2)
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Cascade – ACH range
Slide 6
ACH220
ACH16 ACH18 ACH30 ACH70 ACH 72 ACH112 ACH220 ACH230EQ ACH232 DQ ACH500EQ
ACH112
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Transcritical CO2 BHE portfolio
AXP10 AXP14 CBXP27 CBXP52 CBXP112 AXP27 AXP52 AXP112
Capacity kW 2-15 10-35 40-70 40-100 70-250 10-100 30-150 70-300
PED PSmax bar @ Temp °C
154 150
140
150
90 90
90 90
85 90
130 150
130 150
140 150
UL PSmax psig @ Temp °F
2233 400
2233 400
1190 400
1190 400
1160 400
1885 302
1885 302
Coming
soon Released Sept 2009 April 2010 Dec 2014 June 2010 Mar2015
Thermodynamic cycles CO2
31 oC
Subcritical cycle
Desuperheating Condensing
Transcritical cycle Gas Cooling
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-20 0 10 20 30 40 50 60 70 80 100120
10
30
50
70
90
110
100 200 300 400 500 600
Enthalpy (kJ/kg)
Pres
sure
(ba
r)
Simple transcritical cycle PH-chart
1595
1305
1015
725
435
145
Pre
ssur
e (p
si)
43 86 129 172 215 258
Enthalpy (BTU/lb)
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Simple transcritical cycle TH chart
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0
20
40
60
80
100
120
200 300 400 500 600
Enthalpy (kJ/kg)
Tem
pera
ture
(°C
)
( ) ( ) ( ) ( ) C
TwinTgoutTwoutTgin
TwinTgoutTwoutTginLMTD °=
−−
−−−=
−−
−−−= 6.18
10206596ln
10206596
ln
Using LMTD requires: • Constant heat transfer coefficients • Constant physical properties
248
212
176
140
104
68
32
-4
Tem
pera
ture
(°F)
Enthalpy (BTU/lb)
86 129 172 215 258
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Specific Heat
0
5
10
15
20
25
30
35
40
20 30 40 50 60 70
Temperature [°C]
Cp
[kJ/
kg]
75 bar
80 bar
85 bar
90 bar
95 bar
100 bar
Properties for transcritical CO2
Thermal Conductivity
0
20
40
60
80
100
120
20 40 60 80 100
Temperature [°C]
Ther
mal
Con
duct
ivity
[mW
/m,°C
]
75 bar
80 bar
85 bar
90 bar
95 bar
100 bar
Viscosity
0
20
40
60
80
100
120
20 40 60 80 100
Temperature [°C]D
ynam
ic V
isco
sity
[10*
-6 P
a s] 75 bar
80 bar
85 bar
90 bar
95 bar
100 bar
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-20
0
20
40
60
80
100
120
200 300 400 500 600
Enthalpy (kJ/kg)
Tem
pera
ture
(°C
)Simple transcritical cycle TH chart
Internal pinch point
LMTD = 18.6°C
Internal Pinch point
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140
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Tem
pera
ture
(°F)
Enthalpy (BTU/lb)
86 129 172 215 258
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Consequences of pinch point Heat exchanger design
Slide 13
0
20
40
60
80
100
120
0 20 40 60 80 100
Heat Transfer Area (%)
Tem
pera
tur
(°C
)
CO2Water
Split heat exchanger in sections => One –dimensional model required
Local temperatures and physical properties used. Integration of ∆T over Heat Transfer Area gives MTD
Previous example: MTD = 6.9°C
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140
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Tem
pera
ture
(°F)
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Consequences of pinch point Heat exchanger design
Slide 14
0
20
40
60
80
100
120
0 20 40 60 80 100
Heat Transfer Area (%)
Tem
pera
tur
(°C
)
CO2Water
Pinch point use large heat transfer area
Previous example: Real heat transfer area is 2.7 larger than using LMTD !
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Tem
pera
ture
(°F)
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• Heating water from 10 to 65°C (50 to 149°F)
• What is the optimum operating pressure?
• What is the minimum approach temperature for different operating pressures?
• What is optimum COP?
Heat Exchanger Design Water heating
DIR-15-10-2003 -
1
1,5
2
2,5
3
70 75 80 85 90 95 100 105 110
Compressor outlet pressure [bar]
CO
P
Max
100 bar
100 bar,COP = 2,46
90 bar,COP = 2,51
90 bar
COP = (∆hEVAP*m )/ (∆hComp-is*m)
∆hComp-is
80 bar
80 bar,COP = 1,72
∆hEVAP
35 oC
Supercritical refrigeration process Optimum operating pressure From Danfoss
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0
20
40
60
80
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200 300 400 500 600
Tem
pera
ture
(°C
)
Enthalpy (kJ/kg)
Pressure = 77 bar / 1117 psi TCO2out = 32.6°C / 91°F
Consequences of pinch point Minimum CO2 outlet temperature
Twin= 10°C, Twout = 65°C, Pinch point = 1°C Twin= 50°F, Twout = 149°F, Pinch point = 1.8°F
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Tem
pera
ture
(°F)
Enthalpy (BTU/lb)
86 129 172 215 258
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0
20
40
60
80
100
120
200 300 400 500 600
Tem
pera
ture
(°C
)
Enthalpy (kJ/kg)
Pressure = 85 bar / 1233 psi TCO2out = 23.5°C / 74°F
Consequences of pinch point Minimum CO2 outlet temperature
Twin= 10°C, Twout = 65°C, Pinch point = 1°C Twin= 50°F, Twout = 149°F, Pinch point = 1.8°F
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Tem
pera
ture
(°F)
Enthalpy (BTU/lb)
86 129 172 215 258
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0
20
40
60
80
100
120
200 300 400 500 600
Tem
pera
ture
(°C
)
Enthalpy (kJ/kg)
Pressure = 89 bar / 1291 psi TCO2out = 11°C / 52°F
Consequences of pinch point Minimum CO2 outlet temperature
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-4
Tem
pera
ture
(°F)
Twin= 10°C, Twout = 65°C, Pinch point = 1°C Twin= 50°F, Twout = 149°F, Pinch point = 1.8°F
Enthalpy (BTU/lb)
86 129 172 215 258
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Consequences of pinch point CO2 outlet temp and COP
0
1
2
3
4
5
6
0
5
10
15
20
25
30
35
75 80 85 90 95
COP
Tem
pera
ture
(°C)
Pressure (bar)
Min Co2 out
Approach
COP heat
0
1
1
2
2
3
3
4
4
5
5
0
5
10
15
20
25
30
35
75 80 85 90 95
COP
Tem
pera
ture
(°C)
Pressure (bar)
Min Co2 out
Approach
COP heat
0
1
2
3
4
5
0
5
10
15
20
25
30
35
40
75 80 85 90 95
COP
Tem
pera
ture
(°C)
Pressure (bar)
Min Co2 out
Approach
COP heat
Twin= 10°C, Twout = 65°C, Pinch point = 1°C Twin= 50°F, Twout = 149°F, Pinch point = 1.8°F
1088 1160 1233 1305 1378
Pressure (PSI)
104
95
86
77
68
59
50
41
32
Tem
pera
ture
(°F)
1
1,5
2
2,5
3
70 75 80 85 90 95 100 105 110
Compressor outlet pressure [bar]
CO
P
Max
Supercritical refrigeration process Optimum operating pressure
35 oC
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Consequences of pinch point Minimum operating pressure
Required operating pressure to obtain 1°C (1.8°F) approach @ a pinch point of 1°C (1.8°F) for various inlet and outlet water temperatures
75
80
85
90
95
100
105
110
115
50 60 70 80 90
Ope
ratin
g pr
essu
re (
bar)
Outlet water temperature (°C)
10°C
20°C
30°C
Twin
1668
1595
1523
1450
1378
1305
1233
1160
1088
Ope
ratin
g pr
essu
re (P
SI)
/ 50°F
/ 68°F
/ 86°F
122 140 158 176 194
Outlet water temperature (°F)
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Cold out
Consequences of pinch point Multipass design- effective length
1-pass 2-pass 3-pass 4-pass
Multi-pass design • For high Θ−duties • Close approach
High-Θ
Medium-Θ Low-Θ
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Consequences of pinch point Heat exchanger design
• Design calculation requirements • Use TH chart to check that internal
pinch point is positive and determine minimum operating pressure
• Liquid flow rate (temperature program)
• Segmented model required • High accuracy => 100 sections
• Local physical properties
• Using NIST / Refprop
• Multipass calculations with coupling factor
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Installation Avoid excessive connection loads
Mount the heat exchanger on vibration dampers • Reduce load from piping • Reduce load from thermal
expansion • Reduce vibration
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Suction Gas Heaters Bypass arrangement
Adjust thermal efficiency for the SGHX by Installing bypass! • Avoids high suction temperatures • Reduces losses due to pressure drops • Avoid excessive discharge temperatures
100 % flow
∆T 10°C
∆T 20°C 50 % flow
67 % flow ∆T 30°C
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Subcooler
Air Gas Cooler
Low pressure water circuit
CO2 Circuit
Desuperheater
Expansion valve
Compressor T
T
Tap water
Heat recovery system with space heating and closed loop tap water heating
Space Heating
Accumulator tank to balance demand and supply
Reduce scaling by controlling inlet temperature
Closed loop safeguards against excessive DHW temperature
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Some Do’s • Use the TH chart in Refrigeration !
• To have successful transcritical CO2 performance : • Do NOT use LMTD ! Segmented model with local physical
properties must be used • Gas coolers should be long and slim, high-Θ. • Balance first cost and performance by selecting a proper operating
pressure . • Recover heat using indirect circuits
• Minimize and control scaling
• Avoid excessive DHW temperatures
• Use ackumulator tanks to balance demand and supply
• Avoid thermal stresses in piping system to HX’s • Mount heat exchanger on vibration dampers
• Use bypass when appropriate
Thank you for your attention
And now….Air products for CO2 from Alfa Laval
